1. Field of the Invention
The present invention relates to a method of producing a semiconductor device by cutting out separate pieces of semiconductor chips from a semiconductor wafer, and a grinding machine for use in the method of producing the semiconductor device.
2. Description of Related Art
Production processes of a thin semiconductor device such as a semiconductor device for IC chips include a grinding process for grinding an inactive surface (back surface) of a semiconductor wafer with a grinder. This grinding process is carried out prior to a dicing process for dicing a semiconductor wafer to divide it into semiconductor chips. This is because if the grinding process is carried out after the dicing process, the back surface of each semiconductor chip has to be separately ground, which causes the grinding process to take labor.
However, cutting such a thin semiconductor wafer with a dicing saw causes cracking of the semiconductor wafer and chipping of semiconductor chips. For this reason, a semiconductor wafer cannot be made thinner than a limit thickness, which is about 50 μm, by grinding prior to the dicing process.
Accordingly, carrying out the dicing process prior to the process of grinding the back surface of the semiconductor wafer has been recently proposed. That is, after an active surface of a semiconductor wafer is cut with a dicing saw so as to form grooves, a tape for protecting the surface is stuck to the surface of the semiconductor wafer. In this condition, the back surface of the semiconductor wafer is ground by a grinder. This back surface grinding is continued until the surface of the semiconductor wafer subject to grinding (back surface) by the grinder reaches the grooves, when thinned separate pieces of semiconductor chips can be obtained.
The arrival of the surface subject to grinding at the grooves may be detectable based on, for example, a change in a torque current flowing in a motor for driving the grinder. That is, when the surface subject to grinding reaches the grooves, the contact area between the back surface of the semiconductor wafer and the grinder decreases, causing the torque applied from the back surface of the semiconductor to the grinder to decrease. As a result, the quantity of the torque current flowing in the motor for driving the grinder lessens, according to which the arrival of the surface subject to grinding at the grooves may be detected. Additionally, when grinding by using a chemical is performed as in the case of the CMP (Chemical-Mechanical Polishing) process, the arrival of the surface subject to grinding at the grooves may be detected based on a change in the pH level of the chemical.
However, since the change in the torque current or that in the pH level due to the arrival of the surface subject to grinding at the grooves is so little that it is impossible for both of the above-mentioned methods to precisely detect that the surface subject to grinding has reached the grooves. Accordingly, terminating the back surface grinding in response to the detection of the arrival of the surface subject to grinding at the grooves by such methods tends to lead to insufficient or excessive grinding, causing the produced semiconductor chips to scatter in the thickness.
In addition, as another method for detecting the arrival of the surface subject to grinding at the grooves, the method as shown in FIG. 10 is also possible. In this method, a gauge G is pressed against the surface of a semiconductor wafer W subject to grinding to measure (monitor) the thickness between a wafer stage 91 on which the semiconductor wafer W is mounted and the surface of the semiconductor wafer W subject to grinding so as to determine that the surface subject to grinding has reached the grooves when the thickness of the semiconductor wafer W has become a predetermined value. However, due to unevenness in thickness of a surface protection tape 92 stuck to the surface of the semiconductor wafer W, the results of the measurement by the gauge G include a significant margin of error. Accordingly, it is impossible to obtain a desired thickness e.g. 50 μm of the semiconductor chips with high accuracy.